Light emitting diode (LED) lighting is a fast growing industry due to the high efficiency and long life of LEDs. One difficulty of using LEDs stems from the large mismatch between the alternating current (AC) mains voltage, typically in the range of 100 VAC-277 VAC and the voltage of a single LED which is typically on the order of 1-2V. Another difficulty stems from the range of LED voltages as a function of temperature, manufacturer tolerances, and different manufacturer specifications. Still, another difficulty stems from the fact that LEDs are (direct current) DC devices whereas the primary source of power is AC.
The LED voltage mismatch may be reduced by using long series strings of LEDs. However, this only alleviates part of the issue since it is typically not feasible to place so many LEDs in series to match the AC mains voltage. Furthermore, placing devices in series only partly addresses the issue of voltage matching and does not address the issue of AC-to-DC mismatch or LED voltage variation.
A simple, low-cost solution is to place a large value resistor and a high-voltage diode in series with the LED string. However, this solution is very inefficient, has lifetime issues due to the heating of the resistor, and also leads to a very poor utilization of the available LED power due to the extremely high ripple current produced by the LED.
Many AC-to-DC drivers have been proposed and brought to market to address the issues of driving an LED. One such driver is discussed in U.S. Pat. No. 6,304,464 which proposes a flyback converter as an LED driver and represents the power conversion method used in the majority of AC-to-DC LED drivers which are on the market. While this typical type of driver provides a DC voltage to the LED, these driver types suffer from several drawbacks. One drawback of these drivers is the use of limited-lifetime components which gives the driver a much lower effective lifetime than the LED itself. The limited lifetime components include electrolytic capacitors used as the main storage element and optocouplers used in the feedback loop. These low-lifetime components not only reduce the cost-effectiveness of the overall LED solution, but they also limit the applications to use over relatively small temperature variations. A further drawback of these LED drivers is their inability to provide a lighting solution which provides a specific light level across temperature and manufacturing tolerance variations. Typically, LED drivers regulate the voltage across the LED string. The current is therefore determined by the forward voltage drop of the LEDs and the resistance of the LEDs. Small changes in LED voltage can lead to a large change in LED current and consequently to a large change in light output.
High-power drivers, such as those above 75 W in power, usually incorporate power factor correction on the input. Standard power factor correction circuits use either fixed-frequency continuous-conduction-mode pulse-width-modulation or variable-frequency critical-conduction-mode pulse-width-modulation. Fixed-frequency continuous-conduction-mode pulse-width-modulation typically requires expensive controllers, very large inductors, and large EMI filtering components to reduce the noise created at the single pulse-width-modulation frequency. Furthermore, fixed-frequency controllers can have high switching losses since the frequency is held constant regardless of the waveform amplitude. On the other hand, variable-frequency critical-conduction-mode pulse-width-modulation is inefficient due to the very high ripple current produced in the inductor, and therefore also requires large filters to reduce electro-magnetic-interference (EMI).
FIG. 1 shows a typical circuit of a prior art LED driver. This prior art driver contains AC filter 110, diode bridge 120, flyback converter 160, optional DC EMI filter 140, and output LED string 150. Flyback converter 120 contains storage electrolytic capacitor C101, semiconductor switch S101, transformer TX101, output diode D105, output electrolytic capacitor C102, controller C130, and a feedback circuit made up of components U101, Z101, and R101. Some type of energy storage such as storage electrolytic capacitor C101 is required in any LED driver because the output power is DC while the input power is AC pulsating at double the frequency of the input voltage.
Traditional converters use an electrolytic storage capacitor for several reasons including the following: 1) Electrolytic capacitors are relatively inexpensive compared to most other types of capacitors for a given value of the product of capacitance and voltage rating. 2) The large capacitance of electrolytic capacitors allows significant reduction of ripple voltage and can therefore be used to provide a relatively constant output voltage. 3) The small size of electrolytic capacitors provides the ability to make relatively small drivers.
The prior art converter illustrated in FIG. 1 operates as follows: The AC line charges C101 through diode bridge 120 to a voltage equal to the peak of the AC line voltage. The current drawn from the AC line is very large near the peak and trough of the line voltage and is zero otherwise (aside from a small current that may be drawn by AC EMI filter 110). Switch S101 is controlled with constant frequency pulse-width-modulation to charge the magnetizing inductance of transformer TX101 and then discharge the magnetizing inductance of transformer TX101 through diode D105 and output electrolytic capacitor C102. When C102 charges to the target value of output voltage, Z101 begins to conduct and turns on U101 to throttle back the pulse-width-modulation duty cycle through controller 130. The converter thus produces a constant output voltage. LED string 150 can be modeled as a constant voltage drop in series with a resistor, for input voltages that are greater than the LED turn-on voltage. The LED current is thus equal to the difference between the output voltage and the LED string turn-on voltage, divided by the LED equivalent resistance.
While this prior art converter in FIG. 1 offers a very inexpensive alternative to drive LED strings, it also has many limitations and drawbacks. The drawbacks include the following: 1) Output power varies significantly with LED string voltage. The light level will therefore change substantially depending on LED voltage tolerance, LED temperature, and tolerances in the circuit that regulate the output voltage. 2) Electrolytic capacitors C101 and C102 have a very limited lifetime which will typically be much less than the lifetime of the LED string. This lifetime issue can significantly impact the cost-effectiveness of the LED solution to replace other type of lighting, particularly in higher temperature applications where the electrolytic capacitor lifetime will be even lower. 3) Optocoupler U101 also has a limited lifetime causing the same issues as the limited lifetime of the electrolytic capacitor. 4) The electrolytic capacitor and optocoupler will limit operation of the LED driver to indoor applications due to temperature limitations of both parts. 5) The high pulse currents drawn by the input charging circuit cause significant distortion of the input current and are only allowed for small converters (e.g. below 75 W). 6) Isolated converters such as flyback converters tend to have a relatively low efficiency. Most pulse-width-modulation converters that must adjust the output voltage for changes in the input voltage suffer from higher losses compared with converters that do not regulate output voltage versus input voltage.
FIG. 2 illustrates another prior art LED driver. The driver shown in FIG. 2 is similar to the one shown in FIG. 1, except for the addition of power-factor-correction stage 210 formed by components L201, D205, and S201. The controller 230 operates semiconductor switch S201 in such a way as to draw a sinusoidal current from the AC source. Such converters are well known in the industry and used for higher power converters. Addition of the power-factor-correction converter solves only the issue of high pulse currents and distortion in the grid current, without addressing the other issues. Furthermore, typical methods of operating power-factor-correction converters create additional issues.
Specifically power-factor-correction converters are typically operated in one of two basic control methodologies. The first basic control methodology is referred to herein as critical conduction mode, in which the current through switch S201 is ramped up to a current proportional to the input voltage, and then commutated to D205 when the semiconductor switch is turned off. When the current through L201 decays to zero, switch S201 is then turned on again. The net result is an average current through L201 which is proportional to the input voltage. The frequency varies throughout the ac grid cycle. A great drawback to this control method is that the peak-to-peak ripple current through L201 is always twice as large as the instantaneous current that is drawn from the ac grid. Thus, L201 must be designed to saturate at nearly double the value of current at which it would otherwise be designed, there are large losses due to the high ripple current, and the AC EMI filter must be designed to filter out very large differential currents. This method is typically used for relatively low power power-factor-correction converters less than approximately 120 W due to the cost savings that occur from using a diode D205 which may have some recovery losses.
The second basic control methodology is referred to herein as continuous conduction mode. In this method of operation, switch S201 is operated at constant frequency pulse-width-modulation. However, the duty cycle is controlled to cause the current through L201 to be primarily sinusoidal in phase with the AC grid voltage. Some drawbacks to this method of control include the following: relative complexity of the control compared with the critical conduction mode method, similar ripple amplitude near the zero-crossings of the AC grid current compared with the peak of the grid current, thus causing increased harmonic distortion, and substantial EMI noise concentrated at multiples of the pulse-width-modulation frequency.